I would also be inclined to say that our lacking of ability to take responsibility for our own actions and behaviors is another major contributor. After all, it wasn't the poor saps fault, it was the fact that his mommy and daddy let him play Doom when he was a kid. Or because his mom spanked him from time to time. Surely it is the parents fault. If he was white, then it was because he was an angry white guy. If he was black, well then it is because of the whites that he did this. If he was gay, then of course it was the fault of the religious bigots that liter the world with their fifthy, worthless carcasses.
Can't we just take responsibility for our own actions? Maybe the got went bezerk not because of id software, but because there was something terribly wrong. Though I know I am wrong. If the experts says Doom made him do it, then Doom it was.